Adds aconfig flag for Lifetime Extension into NMS

Bug: 299448097
Test: compiles. only adding flag
Change-Id: I574f4ca05162fc45bd2ed18b5900741bcc692409
diff --git a/packages/SystemUI/aconfig/systemui.aconfig b/packages/SystemUI/aconfig/systemui.aconfig
index 18117a8..2509cfd 100644
--- a/packages/SystemUI/aconfig/systemui.aconfig
+++ b/packages/SystemUI/aconfig/systemui.aconfig
@@ -21,3 +21,11 @@
         "(containing the \"Clear all\" button). Should not bring any behavior changes"
     bug: "293167744"
 }
+
+flag {
+    name: "notification_lifetime_extension_refactor"
+    namespace: "systemui"
+    description: "Enables moving notification lifetime extension management from SystemUI to "
+        "Notification Manager Service"
+    bug: "299448097"
+}